Armoured vehicles are designed to perform various operations while encountering specific threat conditions. In performing these operations, both the payload of the vehicle and the threat conditions that the vehicle encounters can vary dramatically. The payload and threat conditions often correspond to each other since various protection systems are installed on the vehicle depending on the particular theatre of operation. Vehicles are often loaded to the maximum allowed weight rating when the threat is large and have very little payload when there is no threat, such as during a training exercise. For safety considerations, it is important to not exceed the weight rating of the vehicle since critical subsystems could be affected, such as the brake system, the suspension system, the driveshaft, or the wheels.
The vehicle-rated payload may be defined as the difference between the gross weight and the empty weight. Payload is carried by the vehicle chassis as sprung mass and supported by the vehicle suspension system. Unsprung mass may be defined as the weight that is not carried by the suspension system, such as the suspension system itself, the axles and the wheels.
The purpose of a suspension system is to absorb dynamic forces of the sprung mass while the vehicle is in motion to ensure reliable and safe ride performance. This is particularly important for armoured vehicles in severe cross-country terrain and during cornering and transient steering maneuvers on hard surfaces. There are generally three types of suspension systems: passive, semi-active, and active. Passive systems are simple and cost-effective because they are based on fixed spring and damping rates that are optimized for one particular payload, but are incapable of optimization for variable payloads. Active systems can control both the spring and damping rates, but are expensive, require complex, high-power actuators at each wheel, and add a large amount of mass to the vehicle. The semi-active system disclosed herein, on the other hand, retains the advantages of an active system by allowing for controllable dampers and optimization for variable payloads and ride heights, but without the attendant cost, complexity, power requirements, or large increase in mass.
Suspension systems are typically designed for a particular weight grade and vehicle design. However, deployed armoured vehicles often have a widely varying payload and center of gravity. Further, it has proven difficult for a single suspension system to be utilized in different variants of the same vehicle design. As explained below, it is often beneficial to change the ride height of a vehicle, particularly armoured vehicles, to improve the mobility performance and survivability. A suspension system design for armoured vehicles should be tailored to support a family of vehicles with low- and high-sprung mass and variable ride heights to enable safe and reliable mobile performance under varying road conditions.
Several mathematical models exist to rate the mobility performance of military vehicles for different theatres of operation. One key performance criteria is defined as % No-Go, which characterizes the probability that a specific vehicle at a given weight would be unable to traverse the terrain when deployed to a specific theatre. The % No-Go always increases in relation with an increase in vehicle weight. However, by increasing vehicle ride height, it is generally possible to decrease the % No-Go (i.e., improve one aspect of the mobility performance) of a heavily loaded vehicle. Accordingly, it would be advantageous to provide a controllable suspension system for armoured vehicles that is capable of supporting high ride conditions and a resultant high center of gravity for a heavily loaded vehicle that is to deploy to an operational theatre having severe terrain.
Another important rating for military vehicles is survivability. When subjected to an explosive force (e.g. an improvised explosive device (IED)), the survivability of a vehicle and its occupants is related to the vehicle ride height during the time of explosion. When the ride height is higher, the sprung mass (including the hull and occupants) is farther removed from the source of explosive force. It is generally recognized that the explosive force on the hull of a vehicle is reduced by approximately 5% for each 10 mm increase in the distance between the vehicle hull and the source of the explosive force.
Accordingly, when there is a threat of explosive devices aimed at the vehicle hull and its occupants, or to improve % No-Go when traversing severe terrain, it is desirable to increase the ride height by pumping additional gas in the gas springs. However, by increasing the amount of gas locked into a gas spring chamber, the force-deflection relationship of the spring is changed. The most important result is that the amount of available extension stroke is reduced and the available compression stroke is increased. The effect on the gas spring is that the longer compression stroke as well as the increase in gas mass leads to much higher pressures when the spring is compressed to its maximum stroke. The spring design needs to accommodate the higher pressure as well as a modified end stop design because of the shortened extension stroke due to an increased ride height.
While % No-Go and survivability can be improved by increasing ride height, a low ride height is beneficial for other aspects of mobility performance such as vehicle handling and maneuvering. Increasing the ride height increases the vehicle's center of gravity and the susceptibility of a rollover event. To prevent rollovers, armoured vehicles are generally designed to understeer when cornering at excessive speeds. The speed at which steering is limited is termed the critical understeering speed. Similarly, the critical tipping speed is the speed at which the vehicle would start tipping during cornering at a particular radius, and is indirectly proportional to the vehicle's vertical center of gravity or ride height. It is considered safe design practice to ensure that the critical understeering speed is considerably lower than the critical tipping speed. The suspension and steering design should be harmonized to ensure that the vehicle understeers in a constant radius curve long before the critical tipping speed occurs. Variables that affect understeering include axle spacing and lateral tire characteristics, and understeering is directly related to the fact that multi-axle vehicles have scrubbing (non-steerable) axles. There is a need in the prior art to control understeering for vehicles having a variable ride height. There is also a need to keep the vehicle flat during cornering so that the steering response and lateral traction do not substantially change when the vehicle goes from riding high to riding low, or vice versa.
Another purpose of a vehicle suspension system is to maximize traction between the tires and the road surface. During transient steering maneuvers, braking, or accelerating the vehicle rolls (left/right) and/or pitches (front/back). The rolling and pitching motions of the sprung mass result in patterns of weight transfer between the tires, which can lead to a momentary decrease or loss of traction at one or more tires, which in turn can result in unsafe handling performance. A vehicle with increased ride height is generally subject to greater rolling and pitching motions. Accordingly, there is a need in the prior art for a suspension system capable of controlling the rolling and pitching motions of a vehicle with a variable ride height, and therefore a variable center of gravity, to thereby ensure optimal mobility performance.
Some armoured vehicles are equipped with a height management system that adjusts the ride height of the vehicle by varying the column height of gas contained in each suspension spring under nominal conditions. Nominal conditions may be defined as the height relationship between the sprung and unsprung masses when the vehicle is at rest. The current state of the art is to control the vehicle ride height to a predetermined nominal level regardless of the distribution of wheel loads, vehicle payload, mission profile, vehicle speed, or transient driving conditions. Accordingly, there is a need in the art for a controllable suspension system capable of working in conjunction with a height management system to actively control the damping response at variable ride heights (spring extension strokes) while the vehicle is in motion and under varying conditions. The objective is to ensure that vehicle ride and handling are indistinguishable for various ride heights by utilizing a generic damper algorithm that reacts appropriately to counteract the effects of increased rolling and pitching motions the vehicle experiences when driven at higher ride heights.